Without limiting the scope of the invention, its background is described in connection with analytical measurements and analyte detection of a wide variety of analytes using surface plasmon resonance (SPR) sensors.
SPR is an optical surface phenomenon that has been employed in sensors used in chemical, biochemical, biological and biomedical analysis. Summarized briefly, a surface plasmon is known in the art as a surface charge density wave at the surface of a thin conductive film. The oscillation of free electrons at a conductor-dielectric boundary is affected by the refractive index of the material adjacent to the film. Using a polarized beam of monochromatic light, surface plasmon polaritons can be excited. Resonance occurs when the polarized light is totally internally reflected from the conductive film. The light internally reflected from the film has a minimum intensity at the resonance angle. By detecting the resonance angle, the refractive index of a material adjacent to the film may be determined, which is indicative of other properties of the material. A more detailed description of surface plasmon resonance may be found in the article “Surface Plasma Oscillations and Their Applications,” Rather, H., Physics of Thin Films, 1977.
With conventional sensors using surface plasmon resonance, a thin metal film is usually applied to a flat surface of a glass prism. A light source is tuned to direct incident light at the precise resonance angle. After passing through the prism and being reflected by the metal surface plasmon resonance layer, the light is directed by the prism to detectors. Determination of the resonance angle in conventional sensors having a separate light source, a prism and a detector requires a very high degree of precision in order to manufacture and align the separate optical parts so as to be able to produce accurate measurements. The usefulness of this approach, however, has been limited by system complexity related primarily to mechanical alignment issues.
Many problems associated with earlier sensors were overcome by the development of integrally formed SPR sensors in which some or all of the optical components are disposed inside a sensor housing transparent to radiation produced by the radiation source. In these sensors, the radiation source produces light that passes through the housing and strikes an exterior surface of the housing on which a thin conducting layer is formed. The light reflected from the conducting layer is directed toward an array of radiation detectors. The detector having the minimum output level is associated with radiation rays from the source that have bounced off the thin conducting layer at the “resonance” angle which is a function of the refractive index of the material contacting the conducting layer.